As is discussed in more detail below, a number of different techniques have been developed for controllng the amount of power supplied to an inductive load from AC sources. Three prior techniques, which involve synchronous operation of suitable electronic device switches such as a thyristors or transistors, have been termed AC phase control, "inverse" AC phase control and "symmetrical" AC phase control. The term "phase control" embraces the notion of synchronous switching. All of these methods are varients of the basic technique of Pulse Duration Modulation (PDM) or Pulse Width Modulation, normally associated with pulse carrier systems, but in the context of the control power of AC power, usually denoted as "phase control". These techniques are discussed in more detail below together with the shortcomings thereof insofar as the requirements of the loads of the type with which the invention is concerned, i.e., loads having inductive properties.
As will appear, the present invention is applicable to, but in no way limited to, lighting control systems. Patents of interest in the field include some of my earlier patents, e.g., U.S. Pat. Nos. 4,394,603 (issued on Jul. 19, 1983) and 4,352,045 (issued on Sep. 28, 1982) and the references cited therein as well as U.S. Pat. No. 4,350,935 (Spira et al) and the references referred to in the introductory portions thereof and cited therein. More generally, as noted above, the basic prior art AC power control techniques are also discussed below.